Myocardial ischemia is a leading cause of death in both men and women. Our long-term goal is to identify effective pharmacological/molecular interventions for the treatment of myocardial ischemia. In this application, we will systematically investigate estrogen-initiated rapid action and prolonged effect on cardioprotection and stem cell-based repair following myocardial ischemia by using a new approach of acute post-injury administration of estrogen as a therapeutic intervention, which is different from what has been done in primary prevention studies. Notably, poor survival, homing and engraftment of implanted stem cells greatly hamper a better prognosis of stem cell treatment. An approach to improving stem cell-based repair could be to ameliorate the ischemia-aggravated cardiac environment (thus conducive to stem cell survival) and to facilitate stem cell homing into the injured sites. The estrogen-mediated rapid action and prolonged effect could do so. However, there is no information available regarding the role of estrogen, particularly with a post-injury administration, in modulating stem cell-based therapeutic effect after myocardial ischemia. Based on our preliminary study that post-ischemic administration of 17b-estradiol (E2) increased expression levels of mitochondrial (mito-) connexin-43 (Cx43), mito-caveolin-3 (Cav3), and mito-estrogen receptor (ER)?, elevated Cx43 binding to ERa, and reduced mitochondrial oxidative stress following acute ischemia/reperfusion (I/R), it is possible that estrogen initiates an ER?/Cav3/Cx43 interaction in the mitochondria to mediate acute protection and to ameliorate myocardial environment during ischemia. Meanwhile, stromal cell-derived factor 1? (SDF-1), an important chemokine increased in the ischemic heart, plays a critical role in attracting stem cells into the infarcted myocardium. We have indicated that estrogen was responsible for the upregulated SDF- 1 via ER? in female hearts after I/R, which was found to be associated with increased stem cell accumulation in the ischemic myocardium. Therefore, we hypothesize that estrogen-initiated rapid action on mitochondria conveys the improved myocardial environment via mito-Cx43 and mito-Cav3 (Aim 1), whereas the prolonged effect of estrogen is attributable to estrogen-increased stem cell homing through the SDF-1 signaling (Aim 2). Both rapid action and prolonged effect of estrogen via the ER? activation promote stem cell-derived repair following myocardial ischemia. To accomplish the proposed studies, we will utilize mice with inducible cardiac- restricted Cx43 knockout (KO) and SDF-1KO, as well as ER?KO mice. By using an inducible cardiomyocyte- specific transgenic mouse fate-mapping approach, we will also identify the potential role of sex and estrogen in modulating stem cell-derived cardiomyocyte replacement following myocardial ischemia. Finally, we will evaluate the therapeutic efficacy of post-injury administration of a selective estrogen receptor modulator bazedoxifene in the ischemic myocardium.